Efficiency of potassium and phosphate solubilizing Actinobacteria in wheat plant growth promotion

Soil fertility and plant nutrition require an adequate management of essential macronutrients such as Potassium (K) and phosphorus (P) which are mandatory for plant development. In this context, rock phosphate (RP) has been considered as a natural source for manufacturing P-based fertilizers. However, its poor bioavailability is the major drawback for its direct application in agriculture. Therefore, the exploitation of beneficial microbes as biofertilizers has become of paramount importance in agricultural sector due to their potential role in food safety, quality and sustainable production. In this study, we evaluated the ability of nine Phosphate solubilizing Actinobacteria (P13 - P14 - P15 - P16 - P17 - P18 - BC3 - BC10 and BC11) to release P and K from RP and rock potassic (RK) respectively in in vitro and greenhouse conditions. The most performant strains were P18, BC3, BC10 and BC11 since they had not only a broad spectrum of RP solubilization but also had the capacities to solubilize mica as an insoluble source of potassium. Moreover, all strains were able to produce IAA, siderophore, HCN and ammonia and improved significantly the germination rate and the vigor index of wheat. Furthermore, the four efficient strains (P18- BC3 – BC10 and BC11) significantly improved all the agronomic parameters of wheat under RP and RK fertilization in greenhouse conditions namely root length (1.75 – 23.84%), root volume (41.51 – 71.46%), root dry weight (46.89 – 162.41%), shoot length ( 8.92 – 23.56%) and shoot dry weight ( 2.56 – 65.68%) compared to the un-inoculated control. The 16 S rRNA gene sequencing and phylogenetic analysis identified these isolates belong to Streptomyces and Nocardiopsis genera. These findings showed that these strains are promising candidates for the implementation of efficient biofertilization strategies to improve soil fertility and plant yield under Rock P and RK fertilization.